Includes:
- Producing version.cache file if --enable-build-version/NS3_ENABLE_BUILD_VERSION=ON
- Consume version.cache file if the git commit history is not available
- Crash the configuration if neither the git commit history or version.cache file are
available when NS3_ENABLE_BUILD_VERSION=ON
- Warn user to reconfigure the project with NS3_ENABLE_BUILD_VERSION=ON to check
the version
- Only try to remove duplicates if there is a list in find_external_library
